database/sql: fix case where Stmt.Close discards error

Fixes a case where the Stmt.Close() function in database/sql discards any error generated by the Close() function of the contained driverStmt.

Fixes #12798

Change-Id: I40384d6165856665b062d15a643e4ecc09d63fda
Reviewed-on: https://go-review.googlesource.com/15178
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/sql.go

@@ -1576,9 +1576,9 @@ func (s *Stmt) Close() error {
 	s.closed = true
 
 	if s.tx != nil {
-		s.txsi.Close()
+		err := s.txsi.Close()
 		s.mu.Unlock()
-		return nil
+		return err
 	}
 	s.mu.Unlock()